Piezoelectric resonators are used for accurate frequency response and control in many applications, such as for monolithic filters used in radio apparatus. Monolithic crystal filters include a flat quartz wafer having an array of thin film electrodes deposited on each side thereof. The frequency response of such resonators depends upon the thickness of the quartz wafer and the thickness of the electrodes. It is not possible using presently known, low cost, manufacturing techniques to provide the degree of accuracy required in many applications. It has been proposed to modify the characteristics of crystal filters by the use of external devices, but this increases the cost and size of the apparatus and has not been generally satisfactory.
For use at frequencies in the VHF range, the electrodes for the crystal resonators are commonly formed of aluminum. It is known that the mass of aluminum electrodes can be adjusted by anodically growing an oxide thereon. However, a satisfactory method of implementing such anodization has not been known. A liquid anodizing bath has been used, but this has the disadvantage that the resonator must be rinsed and dried before testing, and repeated operations may be required to provide the desired frequency response. It is also known that anodization can be accomplished by use of oxygen plasma produced by a glow discharge, but as it has not been possible to localize this action, high energy plasma must be used. This produces substantial heat so that errors in measurements tend to result, and damage to the crystal and electrodes may take place.